Vacuum heads for disc grinders, sanders and the like



Jan. 24, 1967 VACUUM HEADS FOR DISC GRINDERS 3 Sheets-Sheet 1 Filed July 22, 1964 INVENTOR MARK H. LAND/5' ATTORNEYS M. H. LANDIS Jan. 24, 1967 VACUUM HEADS FOR DISC GRINDERS, SANDERS AND THE LIKE Filed July 22, 1964 5 Sheets-Sheet 2 dwN NN t.

INVENTOR MARK H. LAND/5 BY WMM ATTORNEYS Jan. 24, 1967 M. H. LANDIS 3,299,584

VACUUM HEADS FOR DISC GRINDERS, SANDERS AND THE LIKE Filed July 22, 1964 5 Sheets-Sheet 3 9 .0 a g g rm L n 1 l 1l .o B 8 1 o 1 3 11 v x, I v 00 W' a :EEE L Rm k k .i" 1" ii 1 {WI- 'll II |j Him -l WM HI a ii. Eii': \L\ "Q Q s L on w .n .o IO N O N 00 no n INVENTOR (\1 :0

MARK H. LAND/5 Y/M WMOZM/ ATTORNEYS United States Patent O 3,299,584 VACUUM HEADS FOR DISC GRINDERS, SANDERS AND THE LIKE Mark H. Landis, 228 Philadelphia Ave., Waynesboro, Pa. 17268 Filed July 22, 1964, Ser. No. 384,324 19 Claims. (Cl. 51362) This invention relates to rotatable heads for disc grinders and sanders and like machines. More specifically, this invention relates to rotatable heads in which an abrasive disc is held against a rotatable backing plate by air pressure and in which the disc is deformed into a configuration providing a number of abrading surface portions separated by grooves for receiving particles of the abrasive and dust, chips, or other particles removed from the work by the abrasive.

The abrasive discs of disc grinders and sanders and like machines are conventionally cemented to the backing plate, as exemplified by the construction shown in United States Patent No. 2,556,434 issued June 12, 1951, to F. S. Mitchell for Sanding Disc. This mode of attachment has several disadvantages. First, changing abrasive discs is a time consuming operation as the backing plate must be removed from the spindle or shaft to which it is fixed. Then the worn disc is removed, the cement cleaned from the backing plate, and a new disc cemented to it. The backing plate and disc are then placed in a press until the cement sets, after which the backing plate must be reassembled to the machine spindle or shaft.

Another disadvantage of abrasive heads of the type just described is that the abrasive discs wear unevenly so that some parts of the disc wear to a point necessitating replacement of the disc while other parts may be virtually unworn. Consequently, a substantial proportion of a conventional cemented grinding disc is never used, but is wasted.

In other heretofore proposed arrangements, the abrasive disc is fixed to the backing plate by rivets, screws, or other fasteners. Examples of this type of device are shown in United States Patents Nos. 2,527,762, issued October 31, 1950, to H. H. Pratt for Abrasive Disk Support, and 2,819,568, issued January 14, 1958, to J. N.

Kasick for Grin-ding Wheel. In this type of arrangement, also, removal and replacement of the abrasive disc is a time-consuming operation, generally requiring removal of the backing plate from the machine.

One primary object of the present invention resides in the provision of novel abrading heads for disc grinders and sanders and like machines in which the abrasive disc can be more quickly replaced than is possible in conventional arrangements in which the disc is cemented or attached by rivets, screws, or other fasteners to the backing plate. 7

This and other important objects are attained by a novel abrasive head construction in which ambient air pressure holds the disc against and prevents it from rotating relative to its backing plate. According to the principles of the present invention this is done by employing a novel backing plate provided with a series of uniformly arranged, radially extending spiral grooves which are closed at their inner ends, communicate with the ambient atmosphere at their outer ends, and open onto the disc supporting face of the backing plate.

As the head rotates, air is exhausted from these grooves; and the pressure of the ambient air, acting on the exposed surface of the abrasive disc, deforms the disc into the grooves, which prevents it from rotating relative to the backing plate. When the head stops, the grooves fill with air until the pressure on both sides of the disc is equal. This releases the disc from the back- 3,299,584 Patented Jan. 24, 1967 ing plate, allowing it to be quickly removed and replaced.

Another important advantage of the novel abrasive heads I have invented is that they have a more efficient abrading action than those of conventional construction and, in addition, make it possible to utilize the entire abrasive surface of the disc. Specifically, in conventional abrasive heads, the abrasive head is supported by the backing plate in a flat or planar configuration. The cutting efficiency of such a disc is relatively low; and, in addition, such discs tend to clog with particles of material removed from the article being abraded and particles of abrasive.

In my novel abrasive heads, in contrast, the abrasive disc is deformed into a ribbed configuration with radially oriented portions of the disc alternately depressed into the grooves in the backing plate and supported on the radial lands between adjacent grooves. This provides a series of abrading surfaces which have an intermittent shearing action on the article being ground, an abrading action which is much more efiicient than that provided by a conventional fiat disc.

The use of a grooved backing plate to provide a ribbed disc capable of providing a shearing action is broadly suggested in United States Patent No. 494,471, issued March 28, 1893, to F. N. Gardner for Grinding and Polishing Wheel and Art of Manufacturing Same. In the Gardner construction, however, the abrasive disc is cemented to the backing plate. Therefore, for the reasons discussed above, the removal and replacement of worn discs is a complicated and time-consuming procedure. Moreover, in the Gardner construction, a large portion of the grinding disc, including that depressed in the grooves of the backing plate, is unusable.

In my invention, in contrast to the Gardner construction, the abrasive discs are easily removed and replaced. In addition, the disc can be readily shifted in relation to the backing plate, exposing fresh abrading surfaces. As a result, the entire abrading surface of the abrasive disc is usable.

Rotatable abrasive heads in which the abrasive disc is, at least in part, held against the backing plate by air pressure have also heretofore been proposed. Exemplary of the prior art constructions are those shown in United States Patents Nos. 617,233, issued January 3, 1899, to A. C. de Wolfe for Sandpapering Machine; 1,353,966, issued September 28, 192 0, to W. A. Lorenz for Suction Head for Disk Grinding Machines; 1,355,345, issued October 12, 1920, to W. A. Lorenz for Disk Grinding Machine; and 1,765,808, issued June 24, 1930, to F. I. Tone for Surfacing Apparatus. In comparison to these previously known constructions, my novel invention has a number of important advantages. For example, the Wolfe and Lorenz arrangements employ a fastener to rigidly connect the disc to its backing plate. As discussed above, I have eliminated such rigid connections in my invention, making it possible to more readily remove and replace the abrasive disc and also making it possible to quickly shift the disc relative to its backing plate to expose fresh portions of the grinding disc so that the entire surface of the disc is usable.

Moreover, my invention is capable of producing a higher vacuum and a greater holding force than the relatively inefiicient arrangements disclosed by De Wolfe and Tone. Also, all of the devices disclosed in the aboveidentified patents are relatively complicated in comparison ments disclosed by Lorenz and Tone is that air is exhausted from the grooves in the backing plate only when the abrasive head is rotating. When the head stops, the vacuum in the grooves is automatically broken, releasing the abrasive disc and permitting it, without further steps, to be removed from or shifted relative to the backing plate. Therefore, my invention eliminates the vacuum breaking step which would be required in abovernentioned prior art constructions before the abrasive disc could be removed. Also, in the present invention, replacement of an abrasive disc does not require removal of a fastener (if not the entire abrasive head) as would be necessary in the Lorenz arrangements.

From the foregoing, it will be apparent that another primary object of the present invention is the provision of novel abrasive heads for disc grinders and sanders and the like in which the entire abrasive surface of the abrasive disc is usable.

Another important object of the present invention is the provision of novel abrasive heads for disc grinders and sanders and the like in which an abrasive disc is held against a backing plate by air pressure while the disc is rotating and in which the holding force is auto matically released when the head stops rotating, thereby permitting it to be easily removed from or shifted relative to the backing plate.

Other objects of the present invention include the provision of novel abrasive heads for disc grinders and sanders and the like in accord with the foregoing objects, which:

1) Have a more efficient abrading action than those of the prior art;

(2) Do not require the separate vacuum source of prior art constructions;

(3) Provide a higher vacuum and therefore greater holding force than the prior art constructions which do not utilize a separate vacuum source;

(4) Utilize abrasive discs that can be replaced without removing the head from the machine and can be removed or shifted relative to the backing plate without loosening or removing rivets, screws, or other fasteners;

(5) Have a novel and simple arrangement for centering the abrasive disc on the backing plate;

(6) Are materially simpler than those abrasive heads heretofore proposed in which air pressure is utilized to hold an abrasive disc against a backing plate; and

(7) Are adaptable to profitable manufacture and economical to maintain and use.

Additional objects and further novel features of the present invention will become more "fully apparent from the appended claims and as the ensuing detailed description and discussion proceeds in conjunction with the accompanying drawing, in which:

FIGURE 1 is a plan view of a novel backing plate for an abrasive head for disc grinders and sanders and like machines, constructed in accord with the principles of the present invention;

FIGURE 2 is a partially sectioned view of the backing plate of FIGURE 1, looking in the direction of arrows 22 of the latter figure, and an elevation of an abrasive disc which, together with the backing plate, constitutes an abrasive head constructed in accord with the principles of this invention;

FIGURE 3 is a plan view of a second form of backing plate;

FIGURE 4 is a view, similar to FIGURE 2, of an abrasive head employing the backing plate of FIGURE 3;

FIGURE 5 is a plan view of a third form of backing plate; and

FIGURE 6 is a view similar to FIGURES 2 and 4 of an abrasive head constructed in accord with the principles of the present invention and employing the back ing plate of FIGURE 5.

Referring now to the drawing, FIGURES 1 and 2 illustrate a rotatable abrasive head 20 constructed in accord with the principles of the present invention and composed of an abrasive disc assembly 22 and a backing plate assembly 24.

Abrasive disc assembly 22 comprises a conventional flat abrasive disc 26 consisting of a backing of cloth, kraft paper, or the like to which garnet, sand, aluminum oxide, or other abrasive is bonded by a suitable binder. A flat, circularly sectioned centering disc or wafer 28 is fixed to the center of disc 26 as by a screw 30 to facilitate the mounting of disc assembly 22 on backing plate assembly 24.

Backing plate assembly 24 includes a round backing plate 32 of uniform thickness and a hub 34 fixed to one side of the backing plate by a uniformly spaced series of fasteners such as the illustrated Allen head screws 36. Hub 34 has an internally threaded central bore 38 by which the backing plate assembly 24 of abrasive head 20 is attached to the spindle or shaft of the grinding, sanding, or like machine with which rotatable head 20 is to be employed.

As is best shown in FIGURE 1, a series of uniformly arranged, spirally configured, radially extending grooves 40 are formed in backing plate 32. Grooves 49 have closed inner ends 42 which are concentric with the center of rotation 44 of rotatable head 20 and are reversely curved; i.e., the grooves are convex with respect to the direction of rotation of head 20 (which is identified by arrow 46 in FIGURE 1). The outer ends of grooves 49 open onto the periphery 48 of backing plate 32; and the grooves also open onto the face 50 of backing plate 32 against which abrasive disc 26 rests when abrasive disc assembly 22 is assembled to backing plate assembly 24, as shown in phantom lines in FIGURE 2. As best shown in FIGURE 2, the depth of grooves 40 is somewhat less than half the thickness of backing plate 32.

Referring again to FIGURE 1, the portions of backing plate 32 between groove-s 40 form a series of uniformly arranged, backwardly curved, radially extending lands 52, which extend from the central portion of backing plate 32 to its periphery 48. Over the major portion of their length, lands 52 are substantially equal in width to grooves 40. Adjacent the periphery 48 of backing plate 32, however, the leading edge 54 of each land 52 is first forwardly and then reversely curved, providing a generally triangularly shaped outer end portion 56 on each land which operates as a fan blade when backing plate 32 is rotated.

A circular aperture or recess 58, which opens onto disc supporting surface 50, is formed in the center of backing member 32. This aperture has substantially the same diameter as the centering disc 28 fixed to the abrasive disc 26, the two dimensions preferably being so coordi nated that there is a finger press fit between centering disc 28 and the walls of recess 58 when abrasive disc assembly 22 is assembled to backing plate assembly 24.

Abrasive disc assembly 22 is assembled to backing plate assembly 24 by moving the former in the direction indicated by arrows 60 in FIGURE 2 until centering disc 28 is seated in recess 58 of the backing plate and abrasive disc 26 abuts the supporting surface 50 on lands 52 of backing plate 32 as shown in phantom lines in FIGURE 2. The grinding, sanding, or like machine to which head 20 is assembled is then started, rotating the head in the direction shown by arrow 46. As the head picks up speed abrasive disc 26 is first flattened and held against backing plate 32 by centrifugal force. Then, as the speed of head 20 increases, fan blades 56 exhaust the air from grooves 40 so that the air pressure acting on the surface 62 of disc 26 engaging backing plate 32 is lower than the air pressure acting on the opposite, exposed, abrasive surface 64 of the disc. The greater pressure acting on surface 64 of the disc deforms the disc into grooves 40; and this force, aided by the engagement between the leading edges 54 of land 52 and disc 26, holds the abrasive disc against and prevents it from rotating relative to backing plate 32. Thus, abrasive disc 26 is held against and fixed for rotation with backing plate 32 without the necessity of-cementing it to the backing plate or attaching it to the backing plate with screws, rivets, or other fasteners as in conventional abrasive heads and without the necessity of employing a separate vacuum source as has heretofore been necessary in proposed abrasive head constructions in which the abrasive disc was held against a backing plate by air pressure.

Abrasive disc assembly 22 may be removed from backing plate assembly 24 as quickly as it is installed. To remove the abrasive disc, the machine to which head 20 is attached is turned off, stopping head 20. Air then flows back into grooves 40 until the pressure on both sides of disc 26 is equalized. There is, at this point, no force holding abrasive disc 26 against the backing plate except the slight frictional force between centering disc 28 and the walls of recess 58 so that disc assembly 22 may be removed simply by pulling it off the backing plate.

In addition to ease of installing and removing abrasive discs, there is another extremely important advantage of the novel abrasive head construction just described; viz., a more efiicient abrading action that is obtained from the conventional abrasive head. Specifically, the only portions of abrasive disc 26 which contact the article being abraded are the portions supported on lands 52 of backing plate 32, which constitute only about 50% of the total disc area. Therefore, the article being ground is intermittently contacted by a series of narrow abrading surfaces as abrasive head 20 rotates. This provides a shearing action which is much more efficient than the abrading action provided by a conventional abrasive head in which there is continuous and large area contact between the abrasive disc and the article being abraded. In addition, because of the intermittent shearing action, much higher grinding speeds than were heretofore possible can be attained. Conventional grinders normally operate at speeds ranging from 6500 feet per minute up to a maximum of 8500 feet per minute. Grinding heads constructed in accord with the principles of the present invention can readily be operated at speeds of 9900 feet per minute and higher, as demonstrated in actual tests.

Another major advantage of the novel abrasive head construction just described is that it is substantially less subject to clogging by particles of material removed from the article being abraded and by particles of abrasive loosened from the abrasive disc than a conventional abrasive head in which the disc is supported in a flat configuration. This is because, in the novel construction just described, such particles fall into the depressed, radially extending depressions of disc 26 coincident with grooves 40 and are then thrown off the disc by centrifugal force.

In conjunction with the foregoing, a further important advantage f the construction just described is that, when the exposed abrading surface portions of the disc on lands 52 become worn, disc 25 can be shifted relative to backing plate 32. exposing fresh, unworn portions of the abrasive disc. This is accomplished simply by stopping head 20, which releases disc assembly 22 from backing plate 32, and shifting disc 26 slightly relative to the backing plate (or bypressing the articlebeing abraded against the rotating disc with suflicient pressure to cause the disc to slip slightly relative to the backing plate). The importance of this advantage is apparent when it is realized that, in conventional cemented arrangements, the disc cannot be shifted relative to the backing plate at all and that, in other conventional arrangements employing fasteners to hold the disc on its backing plate, the disc can be shifted only by stopping the machine and going through the timeconsuming procedure of removing or loosening the fasteners, shifting the disc, and then replacing or retightening the fasteners. v

In conjunction with the foregoing, another advantage of the present invention is that, because the abrasive disc can readily be shifted relative to the backing plate, the entire abrasive surface of the abrasive disc is usable. Thus, my

invention provides greater operating economy than conventional abrasive heads in which, as discussed above, only about one half of the abrasive surface is usable. Therefore, in my novel construction, an abrasive disc is capable of removing approximately twice as much material over its useful life as a similar disc of a conventional abrasive head.

The embodiment of my invention illustrated in FIG- URES 3 and 4 is, in many respects, identical to the embodiment of FIGURES 1 and 2. To the extent that the components of the embodiment of FIGURES 3 and 4 are like those of the embodiment just described, like reference characters followed by the letter a will be employed to describe like parts.

The distinction of importance between the embodiment of FIGURES l and 2 and that of FIGURES 3 and 4 is that, in the latter, backing plate 32a is formed from a resilient material such as rubber and has a frustoconical configuration. This provides an abrasive head which is particularly useful for contour grinding and sanding because backing plate 3211 will deform to match the contour of curved surfaces.

To properly support the flexible resilient backing plate 32a, hub 34a is configured so that it extends through a centrally located aperture 66a in the backing plate and terminates in the plane of the latters disc supporting surface 50a. Also, hub 34a is provided, intermediate its ends, with a normally extending circular flange 68a received in a correspondingly configured annular recess 70a in backing plate 320 :to support the central portion of the backing member.

In this embodiment of my invention, the centering recess 58a into which the centering disc 28a of abrasive disc assembly 22a is pressed is formed in the central portion 72a of hub 34a rather than in the backing plate itself as in the embodiment of FIGURES 1 and 2. Also, in this embodiment of my invention, I have shown the use of slotted head screws 36a rather than Allen head screws to assemble backing plate 32a to hub 34a and a slightly different configuration of the outer end portion 56a of lands 52a. These changes, however, do not affect the operation of my invention, but merely exemplify variations in physical form that may be made without departing from the precepts of my invention.

FIGURES 5 and 6 illustrate yet another form of the invention disclosed herein. Again, like reference characters are employed to describe like parts except that those identifying this form of my invention are followed by the letter b.

In this embodiment, backing plate 32b is similar to the backing plate 32 employed in the embodiment of FIG- URES l and 2 except that grooves 40b terminate adjacent but inwardly of the backing plates periphery 48b in closed ends 74b. In addition a series of equally spaced apertures 76b are drilled through backing plate 32b in each of the grooves 40b, providing communication between the grooves and the back side 78b of the backing plate.

In this embodiment of the invention, a flat support disc 80b of substantially the same diameter as backing plate 32b is assembled in parallel spaced relation to the backing plate, Fastened between support disc 80b and backing plate 32b, as by rivets 82b, are a series of fan blades 84b of generally elliptical cross section. Backing plate 32b, fan blades 84b, and support disc 80b constitute a Sirocco fan which, when the head is rotating, exhausts air from grooves 4011 through apertures 76b.

This mode of exhausting the air from grooves 40b is extremely efficient and forms a high vacuum between 70 disc 26b at pressures as high as 12.5 pounds per square inch without abrasive disc 26b slipping relative to backing plate 32b. This means that the etliciency of this embodiment in exhausting the air from grooves 401? is approximately 85% of the theoretical maximum which would be obtained if a perfect vacuum were created in the grooves.

Referring now specifically to FIGURE 6, the assembly of supporting disc 80b, backing plate 32b, and fan blades 84b is fastened to hub 34b of the backing plate assembly 24b by an internally threaded nut or spacer 86]) disposed between the backing plate and support disc. Hub 34b has an annular, normally extending shoulder 88b and a threaded end portion 0b, which extends through a central aperture 92b in support disc 80b and is threaded into spacer 86b, clamping support disc 8% between shoulder 88b and the spacer.

In this embodiment, as in the embodiment of FIG- URES l and 2, the centering recess 58b is formed in the backing plate.

In a series of experiments with a rotatable abrasive head of the type illustrated in FIGURES and 6, a bar of 1015 steel having a cross section of four square inches was pressed against the abrasive disc with a pressure of 50 pounds.

The disc was 18" in diameter, with an effective grinding diameter of 16'', and was rotated at 2350 r.p.m. with a grinding speed of 9844 feet per minute. The abrasive cloth was aluminum oxide, 36 grit; and the disc was driven by a 15 horsepower motor.

Under these conditions, the abrasive disc removed four cubic inches per minute from the steel hat without breakdown of the abrasive disc.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is:

1. A rotatable head for a disc grinder or sander or the like, comprising: an abrasive disc and a backing plate assembly including a backing plate having a generally flat face for supporting said disc, cooperating means fixed to said disc and formed in said backing plate assembly for centering said disc relative to said backing plate, and means for holding said disc against said backing plate and for preventing relative rotation between said disc and said plate consisting solely of a series of uniformly arranged spiral grooves formed in and opening onto the face of said backing plate, said grooves being concentric with the center of said plate and extending to at least adjacent its periphery and means establishing communication between the peripheral terminus of each groove and the ambient atmosphere, whereby said grooves and said disc cooperate to form channels closed at their inner ends and communicating with the ambient atmosphere at their outer ends from which air is exhausted by the rotation of said head to thereby cause the ambient air pressure to hold said disc against the backing plate.

2. A rotatable head for a disc grinder or sander or the like, comprising: an abrasive disc and a backing plate assembly including a backing plate having a generally flat face for supporting said disc; means for centering said disc relative to said backing plate with said disc against said supporting surface without restricting rotation of said disc relative to said backing plate, said last named means including a central aperture in the backing plate assembly opening onto the disc supporting face of the backing plate, a centrally located wafer having slightly smaller dimensions than said aperture fixed to said disc and adapted to be received in said aperture to center said disc relative to said backing plate; and means for exhausting air from between said disc and said backing plate as said head rotates, whereby the ambient air pressure will force said disc against the backing plate and prevent rotation therebetween.

3. A rotatable head for a disc grinder or sander or the like, comprising: an abrasive disc and a backing plate having a generally flat face for supporting said disc, means for centering said disc relative to said backing plate with said disc against said supporting surface without restricting rotation of said disc relative to said backing plate, and means operative incident to the rotation of the backing plate for preventing relative rotation between the backing plate and the disc.

4. A rotatable head for a disc grinder or sander or the like, comprising: an abrasive disc and a backing plate having a generally flat face for supporting said disc, means for centering said disc relative to said backing plate with said disc against said supporting surface without restricting rotation of the disc relative to the backing plate, and means operative incident to the rotation of said backing plate to deform said disc into a configuration providing a series of substantially uniformly spaced apart abrading surfaces extending from the center of said disc to its periphery and to prevent rotation of said disc relative to said backing plate, said last-named means being effective upon the stopping of said backing plate to release said disc for rotational movement relative to said backing plate to thereby expose fresh portions of said disc.

5. A rotatable head for a disc grinder or sander or the like, comprising: an abrasive disc and a backing plate having a generally fiat face for supporting said disc, means for centering said disc relative to said backing plate with said disc against said supporting surface without restricting rotation of said disc relative to said backing plate, and means operative onlywhile said head is rotating toprevent rotation of said disc relative to said backing plate and to deform said disc into a configuration providing a series of substantially uniformly spaced apart abrading surfaces extending from the center of the disc to its periphery comprising uniformly arranged lands and grooves in said plate extending from the central portion of said plate to at least adjacent the periphery thereof with said grooves being closed at their inner ends and opening onto said supporting surface and means establishing communication between the peripheral terminus of each groove and the ambient atmosphere through other than the sup porting surface of said plate, said grooves and said disc together forming a series of Channels closed at their inner. ends and communicating with the ambient atmosphere at their outer ends from which air is exhausted by the rotation of said head to create a reduced pressure in said grooves, whereby the ambient air pressure deforms said disc into saidgrooves to hold said disc against rotation relative to said plate and against said lands to provide said.

shearing surfaces and the stopping of said head releases said disc for rotational movement relative to said head to permit fresh portions of said disc to be shifted onto said lands, thereby providing fresh abrading surfaces.

6. A rotatable head for a disc grinder or sander or the like, comprising: an abrasive disc and a backing plate for supporting said disc having a face with a series of uniformly arranged lands extending from the central portion of the plate to its periphery formed thereon, means for centering said disc relative to said backing plate withsaid disc against said lands without restricting rotation of said disc relative to said plate, and means effective incident to the rotation of said head to depress said disc between said lands to thereby prevent said disc from rotating relative to said backing plate and to provide a series of uniformly spaced abrading surfaces, said last-named means being effective upon the stopping of said head to release said disc for rotational movement relative to said head, whereby said disc may be shifted relative to saidhead to provide fresh abrading surfaces.

7. A backing plate assembly for supporting an abrasive disc in a disc grinder or sander or the like, comprising: a backing plate having a flat circular face, a series of spiral, radially extending grooves formed in said plate, said grooves being continuous from the central portion of the backing plate to at least adjacent the periphery thereof and being spaced apart to provide radially extending lands of substantially the same width as said grooves therebetween, said grooves being closed at their inner ends and communicating with the ambient atmosphere at their outer ends, and means operative only during rotation of said backing plate for exhausting air from said grooves.

8. A backing plate according to claim 7, together with a centering recess formed in said backing plate assembly and opening onto the flat circular face of the backing plate.

9. A backing plate assembly for supporting an abrasive disc in a disc grinder or sander or the like, comprising: a backing plate having a flat circular face, a series of spiral, radially extending grooves formed in said plate, said grooves being continuous from the central portion of the backing plate to at least adjacent the periphery thereof and being spaced apart to provide radially extending lands of substantially the same width as said grooves therebetween, said grooves being closed at their inner ends and communicating with the ambient attmosphere at their outer ends, and means operative only during rotation of said backing plate for exhausting air from said grooves and comprising fan blades integrally formed on the outer ends of said lands.

10. A backing plate assembly according to claim 9, together with a hub fixed to the backing plate, said hub having a flange engaging the backing plate and a central bore with means therein tor attaching the backing plate assembly to the shaft or spindle of a grinding, sanding, or like machine.

11. A backing plate assembly for supporting an abrasive disc in a disc grinder or sander or the like, comprising: a flexible backing plate having a flat circular face, a series of spiral, radially extending grooves formed in said plate, said grooves being continuous from the central portion of the backing plate to at lea-st adjacent the periphery thereof, and being spaced apart to provide radially extending lands of substantially the same width as said grooves therebetween, said grooves being closed at their inner ends and communicating With the ambient atmosphere at their outer ends, and means operative only during rotation of said backing plate ior exhausting air irom said grooves.

12. A backing plate assembly according to claim 11, together with a rigid hub fixed to said backing plate, said hub having a cylindrical portion extending through a central aperture in the backing member and terminating substantially in the plane of the circular race of the backing plate and a circular flange extending normally from the cylindrical portion, the periphery and one side of said flange being in juxtaposition to said flexible backing plate.

13. A backing plate assembly according to claim .11, wherein said hub has a central bore with means therein for attaching the backing plate assembly to the shaft or spindle of a grinding, sanding, or like machine and a centering recess in said hub and opening onto the end of said hub in the plane of the circular face of the backing plate.

.14. A backing plate assembly for supporting an abrasive disc in a disc grinder or sander or the like, comprising: a backing plate having a flat circular face, a series of radially extending, unifiormly arranged, spiral grooves formed in said plate and opening onto said flat circular face, apertures communicating between the outer ends of said grooves and the opposite side of said plate, and fan means for exhausting air from said grooves through said apertures only during rotation of the backing plate.

15. A backing plate assembly according to claim 14, wherein said fan means comprises a supporting disc fixed in parallel spaced relationship to the backing plate and fan blades fixed to and extending between the backing member and supporting disc adjacent said apertures.

16. A backing plate assembly according to claim 14, wherein there is at least one aperture providing communication between each of said grooves and the opposite side of the plate at a point between the inner and outer ends of the groove.

17. A backing plate assembly according to claim 14, together with a shaft having an annular shoulder and an end portion threadedly engaged with the supporting disc with the latter abutting said shoulder.

18. A backing plate assembly according to claim 14, wherein said grooves are continuous from the central portion of the backing plate to a point adjacent the periphery thereof and are spaced apart to provide spirally configured radially extending lands of substantially the same width as the grooves between adjacent grooves, the inner ends of said grooves being closed.

19. A backing plate assembly for supporting an abrasive disc in a disc grinder or sander or the like, comprising: a backing plate having a fiat circular face, a series of spiral, radially extending grooves formed in said plate, said grooves being continuous from the central portion of the backing plate to at least adjacent the periphery thereof and being spaced apart to provide radially extending lands of substantially the same width as said grooves therebetween, and means operative only during rotation of said backing plate for exhausting air from said grooves and for depressing a sufficient portion of said disc into said grooves to reduce the work contacting area of said disc to on the order of of its total area.

References Cited by the Examiner UNITED STATES PATENTS 1,355,345 10/1920 Lorenz 5l-362 2,014,069 9/1935 Jones 51-378 2,991,596 7/1961 Walters 5l-376 ROBERT C. RIORDON, Primary Examiner.

L. S. SELMAN, Assistant Examiner. 

1. A ROTATABLE HEAD FOR A DISC GRINDER OR SANDER OR THE LIKE, COMPRISING: AN ABRASIVE DISC AND A BACKING PLATE ASSEMBLY INCLUDING A BACKING PLATE HAVING A GENERALLY FLAT FACE FOR SUPPORTING SAID DISC, COOPERATING MEANS FIXED TO SAID DISC AND FORMED IN SAID BACKING PLATE ASSEMBLY FOR CENTERING SAID DISC RELATIVE TO SAID BACKING PLATE, AND MEANS FOR HOLDING SAID DISC AGAINST SAID BACKING PLATE AND FOR PREVENTING RELATIVE ROTATION BETWEEN SAID DISC AND SAID PLATE CONSISTING SOLELY OF A SERIES OF UNIFORMLY ARRANGED SPIRAL GROOVES FORMED IN AND OPENING ONTO THE FACE OF SAID BACKING PLATE, SAID GROOVES BEING CONCENTRIC WITH THE CENTER OF SAID PLATE AND EXTENDING TO AT LEAST ADJACENT ITS PERIPHERY AND MEANS ESTABLISHING COMMUNICATION BETWEEN THE PERIPHERAL TERMINUS OF EACH GROOVE AND THE AMBIENT ATMOSPHERE, WHEREBY SAID GROOVES AND SAID DISC COOPERATE TO FORM CHANNELS CLOSED AT THEIR INNER ENDS AND COMMUNICATING WITH THE AMBIENT ATMOSPHERE AT THEIR OUTER ENDS FROM WHICH AIR IS EXHAUSTED BY THE ROTATION OF SAID HEAD TO THEREBY CAUSE THE AMBIENT AIR PRESSURE TO HOLD SAID DISC AGAINST THE BACKING PLATE. 